Aspects of microstructure and texture evolution in the near-u, titanium alloy, Timetal834 were investigated. The electron back scatter diffraction (EBSD) technique was used extensively to acquire microtexture data. To aid analysis of this data, a computer program was written in Visual Basic .NET. The two main functions ofthe program were: (1) to separate the two forms of u phase, primary-u (up) and secondary-u (us) that are present in bimodal microstructures at room temperature, and (2) to reconstruct the parent P phase, which transforms to Us on cooling. The P reconstruction was validated using a metastable titanium alloy, which retains enough P phase at room temperature to allow orientations to be measured directly using EBSD. Parent P orientation maps were successfully reconstructed for several alloys, cooled from above and below the P transus, at a range of cooling rates. The limitations of the method are discussed. Timetal 834 and other titanium alloy forgings can contain regions of closely aligned up grains called macro zones that can have a deleterious effect on fatigue performance. Understanding the origin of macro zones and how they change during processing is an important step in optimising process routes for these alloys. A detailed characterisation was carried out on a slice from a Timetal 834 billet using optical metallography, EBSD and neutron texture analysis. Results showed there was a variation in texture and macro zones through the billet, related to the ingot to billet forging process. The effect of cooling rate on the p~us transformation in a bimodal microstructure was investigated using a Jominy' end quench test. At the highest cooling rates, a martensitic microstructure was observed. At lower cooling rates, a diffusional transformation to WidmansHitten Us, allotriomorphic u at PIP grain boundaries and growth of the existing up grains was observed. Variant selection in Widmanstatten Us was observed at PIP boundaries with specific misorientations and at uplP boundaries with a specific orientation relation. Forging to produce a bimodal microstructure was simulated using laboratory plane strain compression tests at 101OoC. The majority of the strain was accommodated in the P phase. The P phase undergoes dynamic recovery resulting in a subgrain structure. At strain rates above 0.18s-1 , selective recrystallisation in the P phase occurred in the vicinity of PIP grain boundaries and up grains. The up grains deform by slip and additionally by deformation twinning at strain rates above 1.8s-1 Some localised dynamic recrystallisation was observed but the majority of the up phase remained unrecrystallised. Macrozones remained at a strain of 0.63 but were less apparent at a strain of 1.23.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:505440 |
| Date | January 2009 |
| Creators | Davies, Peter Stephen |
| Publisher | University of Sheffield |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://etheses.whiterose.ac.uk/12805/ |
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